This invention concerns digestion of solid organic material, for example solids from sewage waste, in a process of a wastewater treatment plant. In particular, the invention concerns the use of phagotrophic algae in connection with aerobic or anaerobic digestion of solids, especially waste activated sludge (WAS), to more efficiently digest solids, so as to meet over a shorter period of time, volatile solids standards and specific oxygen uptake rate requirements as well as pathogen reduction requirements.
Reference is made to U.S. Pat. No. 3,780,471; Salerno et al., “Biogas Production from Algae Biomass Harvested at Wastewater Treatment Ponds”, ASABE Bioenergy Engineering Conference Presentation, Paper No. Bio098023, Oct. 11-14, 2009; Golueke et al., “Anaerobic Digestion of Algae”, Sanitary Eng. Res. Lab., Dept. of Engineering, Univ. of CA, Berkeley, Calif., Sep. 4, 1956; and PCT Publication WO 2009/088839. The latter publication describes a method to produce an algae product utilizing algae, including phagotrophic algae. The process involves introduction of the algae to a liquid growth medium including microorganisms, so that the algae consume the microorganisms. The liquid growth medium can comprise organic waste products. The end product is a useful, saleable algae product.
Overall, the previous published application (WO 2009/088839), as well as currently pending PCT application published as WO 2014/194174, to which this application is related, pointed toward growing phagotrophic algae by exposing it to various organic waste material including waste activated sludge (WAS, typical by-product of biological wastewater treatment). Pre-treatment before algae growth was also discussed, one pre-treatment being partial breakdown of biological microbe cell structures using ultrasound. Any of the pretreatment options discussed in these published applications can also be used with the current process described below. Also, any of the phagotrophic algae species listed in the published applications are candidates for the processes described herein. In the disclosures of the publications the object and the end result were the production of algal biomass, which could then be applied for other uses. Neither disclosure was directed toward more efficient digestion of algae-rich biomass as an end in itself, which may even include total die-off of algae in some applications. WO 2014/194174 is incorporated herein by reference in its entirety.